REGENERATION 5 



possesses the power of i-eproducing the whole. If, with the help of 

 a bristle, we turn one of these polyps outside in like the finger 

 of a glove, and then prevent it turning right again by sticking the 

 bristle transversely through it, it does not live, but soon dies, 

 obviously because the cells of the two layers of the body, ectoderm 

 and endoderm, cannot mutually replace each other, and cannot 

 mutually produce each other. The inner layer, now turned outwards, 

 cannot resist the influence of the water, and the outer layer, now 

 turned inwards, cannot effect digestion ; in short, one cannot be trans- 

 formed into the other, and we must therefore conclude that both are 

 specialized, that they no longer contain the complete germ-plasm, but 

 only the specific determinants of ectoderm and endoderm respectively. 



The animal's high regenerative capacity must therefore depend 

 on the fact that certain cells of the ectoderm are equipped with the 

 complete determinant-complex of the ectoderm, in the form of an 

 inactive accessory idioplasm, which is excited to regenerative activity 

 by the stimulus of wounding, and that, in the same way, the cells of 

 the endoderm are equipped with the whole determinant-complex of 

 the endoderm. It need not be decided whether all or only many 

 of the cells, perhaps the younger ones, are thus adapted for regenera- 

 tion ; in any case a great many of them must be distributed through- 

 out the whole body, with perhaps the exception of the tentacles, 

 which are by themselves unable to reproduce the whole animal. 

 When the animal is mutilated, the cells of both layers, equipped with 

 their respective determinant-aggregates, co-operate in reproducing the 

 whole from a part. 



It is true that even with these assumptions we only reach the 

 threshold of a real explanation. For, given that all the determinants 

 of the species must be present in a fragment, we are not in a position 

 to show how these set about reconstructing the animal in its integrity, 

 and the most that we can say is, that it must depend on the specific 

 kind of stimulus to which each of the cells is exposed through its 

 direct and more remote environment, which determinants are to be 

 first liberated, and therefore which parts are to be reconstructed. 



That there are at work regulative forces, such as we were already 

 compelled to assume in regard to the division and regeneration of 

 unicellular organisms, as to the nature of which we cannot yet make 

 any definite statement, but which we may call ' polarities,' or, as 

 I prefer to say, ' afiinities,' is shown by countless experiments which 

 have been made, particularly with the freshwater polyp. Thus Rand 

 cut off the anterior end of the polyp with its circle of tentacles, and 

 the excised disk of living substance lengthened in a transverse 



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